Three criteria, all must pass
A conductor is correctly sized only when it satisfies current-carrying capacity, voltage drop and short-circuit withstand simultaneously. Designers who check only ampacity produce systems that pass inspection and then underperform for 25 years.
In PV, voltage drop is usually the binding constraint on DC string runs, while ampacity binds on short AC runs.
DC string and array cable
Design current for a string is 1.25 × Isc,STC per IEC 62548, which covers irradiance enhancement events. For combined array cable, sum the string design currents.
Voltage drop for a two-wire DC circuit: Vdrop = 2 × L × I × ρ ÷ A, where L is one-way length in metres, ρ is resistivity (0.0172 Ω·mm²/m for copper at 20 °C, ~0.0225 at 70 °C), and A is cross-section in mm². Limit DC drop to 1% for string cable and 2% total DC-side.
AC cable from inverter to point of coupling
Single-phase: Vdrop = 2 × L × I × ρ ÷ A. Three-phase: Vdrop = √3 × L × I × ρ ÷ A. Design current is 1.25 × inverter rated AC output current.
Keep total AC drop within 1–2%; IS 732 and IEC 60364-5-52 recommend total installation drop below 5% from origin to load, and the PV contribution should be a small share of that.
Derating factors that people forget
Ambient temperature: rooftop cable trays reach 60–70 °C, giving derating factors of 0.71–0.58 against the 30 °C table baseline.
Grouping: six circuits in one conduit derate to about 0.57. Installation method, thermal insulation contact and direct sunlight each apply further factors. Multiply them — do not take the worst one alone.
Worked example
A string of Isc 13.9 A, one-way length 45 m, 4 mm² copper at 70 °C: I_design = 17.4 A; Vdrop = 2 × 45 × 17.4 × 0.0225 ÷ 4 = 8.8 V. Against a 700 V string voltage that is 1.26% — above the 1% target, so step up to 6 mm² and the drop falls to 5.9 V (0.84%).
Always re-check ampacity after the voltage-drop step: the larger conductor must still fit the connector and terminal range.